3.0 Payload Sensors Subsystem

Transcription

1 3.0 Payload Sensors Subsystem If the C&DH subsystem is the brain of the CubeSat, then the Payload Sensors Subsystem is the eyes and nose of the CubeSat. The payload sensors subsystem consists of several sensors, such as temperature, pressure, accelerometer, and GPS module. 3.1 Background The previous CanSat had only temperature, pressure, and accelerator sensors installed in it. All of these sensors were analog, meaning that they produced a continuous stream of signal. These analog signals were then converted to digital signals via the A/D converter discussed in C&DH subsystem. All the data was stored in SRAM, where the microcontroller converts the data into a telemetry stream and sends it to the communication subsystem. The previous CanSat group had problems with the accelerometer and the temperature sensors. The accelerometer stopped working midway of the flight test, and the temperature sensor gave faulty data, as it was placed near the microcontroller that produced heat during data processing. 3.2 Requirements and Constraints The objective of the payload sensors subsystem is to acquire scientific data as well as monitoring the health and progress of various subsystems of the CubeSat. Again, the payload sensors subsystem should be light in weight, consume a small amount of power, be high in sensitivity, and should be able to produce undistorted analog and digital signals. Moreover, the payload sensors subsystem should have room for extra payload sensors for future projects, and sensors should be easy to remove for repairing.

2 3.3 Options and Evaluations The payload sensors have been chosen and range from $1.00 for a temperature sensor to about $ for a GPS module. Many companies offer these sensors, so careful research was performed before we selected the appropriate ones for the CubeSat. The previous CanSat group purchased approximately five temperature sensors, model number LM 135 (see Figure 5), and used only one; therefore, these temperature sensors are available to Satellite Solutions for free. These temperature sensors have an operating range of -55 C to +150 C with an accuracy of ±1 C over a wide range. Figure 1 shows the picture of the temperature sensor installed near the microcontroller in the CanSat circuitry. Figure 5: Temperature sensor, LM 135.

3 Figures 6, 7, 8, and 9 are pictures of the pressure sensor (MPX4115A), accelerometer (three axis from Motorola), and GPS module (Motorola M12+ Oncore), respectively. Figure 6: Pressure sensor, MPX4115A. Figure 7: Three-axis accelerometer sensor from Motorola.

4 Figure 8: Motorola M12+ Oncore GPS Module. 3.4 Design The payload sensors board will be easiest to fabricate because all the components are ready-made. Satellite Solutions will only have to solder everything onto a circuit board, measuring less than 6.5 x 9.5 centimeters. The temperature sensors will be placed at several places inside the CubeSat (TBD) to monitor the temperature variations for various subsystems. An additional temperature sensor will be placed on the outside surface to measure the ambient atmospheric temperature. The pressure sensor will be placed where the atmospheric pressure can be sensed, possibly on the bottom panel of the CubeSat. The accelerometer will be placed near the geometric center of the structure, closer to the batteries to read the acceleration in all three directions. The GPS module will be placed next to the MaxStream transmitter (see circuit board 1 in Figure 32) and its

5 required GPS antenna on top of the C&DH subsystem circuit board (see circuit board 8 in Figure 32). The main required circuitry for all sensors will be placed on one circuit board (see circuit board 4 in Figure 32). The Payload Sensor subsystem will have room for additional payloads and will be supplied with required controlling commands from CD&H subsystem. The analog output from the sensors will then be converted to SM band signals and will be transmitted to the ground station through the Communication Subsystem. The next section discusses the Communication subsystem in detail.